Advancements in the technology of digital computation are coming at the systems level via programming techniques and at the hardware level through improved circuit technology including processing and packaging as well as logic circuit design.
The logic employed in digital circuits may be either voltage mode or current mode. Heretofore, voltage mode logic (e.g. diode-transistor logic and transistor-transistor logic), wherein the voltage level of the signal imports the content, has been predominant. However, with increases in computer speed now being limited to a large extent by circuit operation time, the inherent delay attendant with transistor saturation in voltage mode logic presents a limiting parameter.
Current mode logic, wherein logic is transmitted through currents, overcomes some of the limitations of voltage mode logic. For example, fewer circuit elements and less supply power is required than for voltage logic. An importantly, "soft-saturating" gates may be employed which permit increased circuit speed.
Disclosed in my copending application Ser. No. 562,316 is a current mode arithmetic logic array which operates on pure binary data. The array employs interconnected soft-saturating current mode logic series gates including a lower level gate and an upper level gate with the output voltage range of the array limited to approximately one-half volt. The array has a number of modes of operation, and to expedite operation on large numbers the array may be used in combination with the carry look-ahead current mode logic array disclosed in my copending application Ser. No. 562,327 filed Mar. 26, 1975.
One limitation of the arithmetic logic array in my copending application Ser. No. 562,316 filed Mar. 26, 1975 is the applicability of the array only with binary data; the array does not operate on decimal coded data. Logic arrays have been proposed heretofore which operate on both binary and binary coded decimal data, such as for example, U.S. Pat. No. 3,711,693, assigned to the present assignee, but this array does not employ current mode logic.